Let me restate some basics another way. Hopefully, this can help those
seeking grounds about (vertical) grounds.
The ideal ground screen under a short or quarter wave vertical is made up of
conductors as long as the vertical and little, if any, longer. The fields
originating from the top of the vertical tend to return to earth as
vertically as possible within the shadow of the vertical's height and extra
length to this, increases capacity to earth and thus loss. The objective is
to keep return currents in the conductors and not in the soil!
The ground under the vertical serves to collect current from infinite field
lines coming down from the tip of the vertical.
Thus, the ideal is a solid sheet of metal lying on the earth, not a buried
conductor!
However, short of using sheet or screen wire, one can emulate those by
applying a radial in each desired direction of transmission. Put in as many
radials as you wish to have directions to transmit.
However, as a practical matter, studies have shown there is little to be
gained going beyond a 3 degree spacing at MF broadcast band, (120 radials).
Other studies have shown that fewer radials are needed to have almost the
same effect at higher frequency (ham) bands. Thus, a six degree spacing,
(60 radials) is nearly top efficiency for the ham bands. IF you are so
located that there are directions of few hams, put in your radials in those
directions where there are stations to be worked.
Decide from the dB gains shown in the literature, for various numbers of
radials and their lengths; what total length of wire you can afford, can
install on site, and then install in such lengths as not to extend beyond
the shadow of your vertical and thus waste the copper's effect.
In considering use of buried vs. surface radials or elevated radials,
consider the corrosive nature of your particular earth location. The same
goes for choice of wire type. You may be able to use thin insulated wire,
and not suffer corrosion while others need heavier wire for durability.
This may also enter in, when considering if your radial field is a high
surface traffic area.
Model studies have shown that vertical dipoles do not benefit from a radial
system under them since they are a complete antenna unto themselves. (see L.
B. Cebik, W4RNL publications, or www.cebik.com) Where they can benefit, is
in having favorable conditions of reflection in their Fresnel Zone, 2 to 5
waves from the antenna itself. However, this area is seldom under the
control of the amateur operator. Local earth conditions may modify the
above statement, but present state of the modeling art does show a dipole is
a complete radiator independent of earth, (save for end coupling capacitance
if too close on that end).
Much of the above with regard to antennas much shorter than a quarter wave
has recently been confirmed in ongoing small antenna research at the
University of Texas Applied Research Labs.
Papers on the FLEX antenna developed there are being published by IEEE
Antennas and Propagation, and Antennas Symposium 2000.
-Stuart
K5KVH
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